Long chain saturated aliphatic amino acids are important intermediates for the production of long chain nylons. Because of their unusual molecular structure, long chain nylons possess extraordinary physical properties, i.e., higher mechanical strength than metal, low hygroscopicity, excellent resistance to oil, low temperature, abrasion, and chemical corrosion, and most importantly, easy to fabricate. Long chain nylons are made into many kinds of plastics products, spun to fibers and stretched to thin films. Long chain nylons are also used in paints and hot melt adhesives. Hence, long chain nylons find wide applications in automobile, electrical, electronic, telecommunications, petrochemical, and aerospace industries.
Long chain amino acids, especially 9-aminononanoic acid and 11-aminoundecanoic acid, are used industrially as a monomer to produce nylong-9 and nylon-11, respectively. In addition, long chain amino acid, 12-aminolauric acid, may be used to produce an industrially important nylon-12, which is currently produced from laurolactam.
For the long chain amino acids to be used in the process to produce polyamide or nylons, they have to be refined to a grade suitable for polymerization. There are several known methods to achieve such purification.
In the first process, long chain amino acids are purified by recrystallization in water. U.S. Pat. No. 2,462,855 describes such a process to refine the crude 1 l-aminoundecanoic acid. In the disclosed process, 53 parts of the long chain amino acid is obtained from 1,500 parts of water. Through the concentration of mother liquor, 4 parts of additional long chain amino acid is recovered. Recrystallization from such dilute solution is not economical and energy-intensive to concentrate the mother liquor.
China Patent Appl. No. 103804209A describes a process to dissolve 11-aminoundecanoic acid in boiling water of ten times the amount of the amino acid to perform recrystallization. In light of the disclosure of U.S. Pat. No. 2,462,855 and the solubility property of the amino acid, it is unlikely for the process to achieve a clear solution so that impurities can be removed by filtration during the recrystallization.
U.S. Pat. No. 2,674,607 describes a method to purify 9-aminononanoic acid by recrystallization from boiling water.
In the second process, long chain amino acids are purified by recrystallization in aqueous ethanol solution. GB 953,621 discloses a process to purify 10-aminodecanoic acid by recrystallization of the crude amino acid in a 80% aqueous ethanol.
U.S. Pat. No. 5,498,733 describes a process to refine 1-aminoundecanoic acid by recrystallizing the crude amino acid first in water, then in aqueous ethanol (3:1 of ethanol:water). The amount of water and aqueous ethanol used in the recrystallization are 122 times and 62 times the weight of the amino acid, respectively.
U.S. Pat. Nos. 5,434,307 and 5,530,148 describe a process to recrystallize 12-aminododecanoic acid in aqueous ethanol (1:1 of ethanol:water) in a recovery yield of 87%.
In the third process, 9-aminononanoic acid is purified by recrystallization in aqueous acetone solution. Kohlhase et al (J. Am. Oil Chemist's Soc., 1970, Vol. 47, pp 183-188) describe a method to recrystallize the crude product of 9-aminononanoic acid in an aqueous solution of acetone (50%) with a recovery yield of 74%. The solvent is 130 times the weight of crude 9-aminononanoic acid.
Perkins et al (J. Am. Oil Chemist's Soc., 1975, Vol. 52, pp 473-477) report an improvement of Kohlhase et al by first dissolving the crude 9-aminononanoic acid in hot water to form a 10% solution, followed by treatment with activated carbon and Cellite. After filtration, the solution is cooled and then mixed with an equal volume of acetone to precipitate the amino acid. Perkins et al further note that the amino acid crystallizes from water or water-acetone mixture as fine hydrophilic crystals, which contain the mother liquor. The crude amino acid has to be recrystallized often twice to obtain a product suitable for polymerization.
Finally, Miller et al (Ind. Eng. Chem. Prod. Res. Develop., 1971, Vol. 10, pp 442-447) have studied in detail the purification of 9-aminononanoic acid by comparing the precipitation process from an aqueous solution by using acetone, tetrahydrofuran, or dimethylformamide. A large quantity of organic solvents is required to precipitate 9-aminononanoic acid.
In general, the process of recrystallization in a suitable solvent is required to obtain a long chain amino acid that is suitable for polymerization to produce polyamide or nylon. Since the solubility of long chain amino acids is small in water and organic solvent, a large amount of water or aqueous solvent is required to dissolve the long chain amino acid. Hence, the processes according to prior art require a large amount of energy to concentrate the mother liquor, to recover the solvent, and are not economical.
It is the object of the present invention to overcome the disadvantage of the known processes for the purification of long chain amino acids and to provide additional advantage, which will become apparent from the following description.
By the process according to the present invention, purification of long chain amino acids by recrystallization can be carried out at high concentration and long chain amino acids of high purity are obtained simply and economically on an industrial scale.